The radio transmission of both analog and digital communications intelligence is normally effected by one of two methods. In one, referred to as an amplitude modulation system, a sinusoidal radio frequency carrier is modulated in amplitude in terms of the intelligence or communications signal, and when the signal is received at a receiving location, the reverse process, that is, demodulation of the carrier, is effected to recover the communications signal. The other system employs what is termed frequency modulation, and instead of amplitude modulation of the carrier signal, it is frequency modulated. When a frequency modulated signal is received, circuitry is employed which performs what is termed discrimination wherein changes in frequency are changed to changes in amplitude in accordance with the original modulation, and thereby a communications signal is recovered. In both systems, there is as a basis a sinusoidal carrier which is assigned and occupies a distinctive frequency band width, or channel, and this channel occupies spectrum space which, if interference is to be avoided, cannot be utilized by other transmissions.
At this time, almost every nook and cranny of spectrum space is being utilized, and there is a tremendous need for some method of expanding the availability of medium for communications. In consideration of this, it has been suggested that instead of the use of discrete frequency channels for radio communications links, which is the conventional approach, a radio transmission link employing a wider frequency spectrum could be divided which may extend over a range of 10 to 100 times the intelligence band width being transmitted, but wherein the energy of any single frequency making up that spectrum be very low, typically below normal noise levels. While it is obvious that this type of transmission would be essentially non-interfering with other services, the applicant is unaware of any available system for practicing this.